As wireless network traffic rises, Radio Access Technologies (RATs) evolve to support higher and higher bandwidths. Modern RATs may satisfy bandwidth requirements with standards established for various frequency bands. For example, new frequency bands may be allocated to provide additional bandwidth. However, often the new frequency bands are allocated at higher frequencies and consequently have smaller coverage areas.
One way to increase the coverage area is to increase the output power of devices. However, increasing the output power of a device reduces the power efficiency of the device. A first approach to mitigate for the decrease in power efficiency is to design a device using: a polar modulator with one Digital-to-Analog Converter (DAC), and a Local Oscillator (LO) that provides a clock signal to the DAC at a minimum rate. The input signal to the Power Amplifier (PA) of the device may vary only in phase. The power supply to the PA is fed through a Direct Current-to-Direct Current (DC-DC) converter. Then, amplitude modulation may be performed by adjusting the gain of the PA. The gain of the PA is adjusted by modulating the supply voltage based on tracking envelope information of the received data. Unfortunately, an envelope tracking power supply requires a feedback mechanism for accuracy. The feedback mechanism increases the silicon area and introduces design problems for supporting higher bandwidth.
A second approach to mitigate for the decrease in power efficiency is to use a polar transmitter in which the PA is directly modulated. The direct modulation of the PA is performed by feeding the entire radius to the PA through the continuous power supply, and by feeding a modulated carrier signal with fixed amplitude to the signal input of the PA. A cellular system has an adjustable gain range of about 80 dB and the PA has gain dispersion. Hence, no current gain values can be set at lower amplitudes. Directly modulating the PA may reduce power dissipation to a certain extent. However, the far off noise of the directly modulated PA is low. As such, the first and second approaches are sub-optimal for transmitters intended to support high bandwidth.
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